rf everywhere

Guest

What is the advantage (if any) to using co-ax v. open-wire feed for
above HFBC example?

Click to expand...

When you have several (sub)megawatt HF transmitters and a few
directional antennas aimed on different continents, you really need a
big switchyard. with coaxial relays. I do not understand, how to do
this with open-wire systems.

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Coax has the fields contained within the cable, open-wire does not.
Structures, people, and critters in proximity to coax don't matter.
It's a different matter with open-wire. Open wire usually requires a
BalUn, as well.

Coax has the fields contained within the cable, open-wire does not.
Structures, people, and critters in proximity to coax don't matter. It's a
different matter with open-wire. Open wire usually requires a BalUn, as
well.

Click to expand...

The WWII, British, CH radar (circa 50MHz), used open wire feeders to the
transmitting antennas. The transmitters were a Marconi-EMI television
design, of a few hundred kW, modified for pulse modulation, one of the
reasons for killing the early British TV service.

The guy I knew who'd worked on it described how the routing at one site,
around the edge of the field, with several corners, to conceal the feeders
from the air, couldn't be made to work, and they had to go direct, making
a beautiful sighting line to the camouflaged transmitter.

Guest

The WWII, British, CH radar (circa 50MHz), used open wire feeders to the
transmitting antennas. The transmitters were a Marconi-EMI television
design, of a few hundred kW, modified for pulse modulation, one of the
reasons for killing the early British TV service.

Click to expand...

Good grief, I didn't say open-wire was never used, just that it is
more difficult.

The guy I knew who'd worked on it described how the routing at one site,
around the edge of the field, with several corners, to conceal the feeders
from the air, couldn't be made to work, and they had to go direct, making
a beautiful sighting line to the camouflaged transmitter.

Click to expand...

Perhaps Coax wasn't available? Seems like a perfect opportunity for a
ruse. ;-)

The original transmitters at that site were made by Crosley. Green
metalflake paint and big glass doors in front of the finals. Beautiful
examples of '30s industrial design, but built before Television. They
tried numerous methods to eliminate the TVI, and were replacing them
with custom designed National radio transmitters. When you changed
frequency, the transmitter reduced power and self tuned every stage.
All 10, or any combination could be fed from one MFO for power levels
from 50 to 500 KW. The self tuning was done with detectors to measure
the signal and large servos to drive the variable capacitors & roller
inductors.

Click to expand...

Once upon a time i had a USC-30, 2 to 30 MHz 1kW transceiver, in my care.
It auto-tuned in the same way. I got to watch it once (with the covers
off) and it was a thing of beauty to behold. It had its own low power (50
W) dummy load and tuned up into that. Then it commanded the auto-tuner
of the selected antenna to tune up. Just beautiful, but 30 seconds for
any frequency change from 100 Hz to 28 Mhz. You used a different receiver
and antenna to find an empty spot in an authorized band to transmit.